The present invention relates to a hydraulic spool valve and, more specifically, to a hydraulic spool valve used in a hydraulic vehicle drive.
Hydraulic spool valves of this type are known in the prior art. One example is shown in the publication “Axialkolben-Motoren Baureihe 90” (“Series 90 Axial Piston Motors”) by Sauer-Sundstrand GmbH & Co., publication number SMF9, 06/92, 699801D. These spool valves serve as a function of the hydraulic fluid pressure in the two lines conducting the hydraulic fluid to cause the hydraulic fluid having the lower pressure to flow out through the outlet bore as soon as the difference in pressure has reached a certain level.
Hydraulic vehicle drives are a typical application of a hydraulic spool valve of this type. Hydraulic vehicle drives typically have a closed hydraulic circuit, which comprises a variable displacement pump, a hydraulic motor driving the drive wheels of the vehicle and two lines which connect the variable displacement pump and the hydraulic motor and are intended for the hydraulic fluid. Since the hydraulic fluid is heated considerably during the normal driving mode, a constant exchange of the hydraulic fluid is required, and the hydraulic motor has to be cooled. When a certain difference in pressure in the forward-flow and return line of the hydraulic motor is reached, the hydraulic spool valve opens and allows a flushing flow to flow from the low-pressure side through a flushing-pressure-limiting valve. This gives rise, during the normal driving mode, to a continuous leakage flow which can amount to approximately 10% of the maximum circulation of the hydraulic fluid in the closed circuit.
Conventional hydraulic spool valves are designed in such a manner that, in the neutral central position of the spool-valve piston, the pre-stressed compression springs on both sides of the spool-valve piston act upon the piston and bear at the same time against stops of the spool-valve housing. As soon as the difference in pressure of the hydraulic fluid in the two inlet bores of the spool valve affect the displacement of the spool-valve piston in the housing, the compression spring, which is pre-stressed in the direction of the displacement movement, is therefore rendered ineffective because it comes to rest against the stop of the spool-valve housing. The spool-valve piston is decoupled to a certain extent from this compression spring, and so only the other compression spring which acts counter to the displacement movement remains effective. The hydraulic fluid which is under relatively high pressure therefore has to overcome the forces acting in the opposite direction on the spool-valve piston and originate from the compression spring, which still remains effective, and the hydraulic fluid having the lower pressure.
Because of this, most applications generally operate with high pressures and powerful compression springs. In the case of flushing spool valves, the spring pre-stressing forces corresponding to an opening pressure from the neutral central position typically are in the range of 7 to 16 bar. Under selected operating conditions, the hydraulic spool valves according to the prior art operate satisfactorily. Difficulties arise, however, if the same hydraulic spool valves are also intended to operate as a high-pressure and low-pressure side when there are small differences in pressure and frequent changing of the inlet bores. This may have the effect of a spool valve of this type remaining in its neutral central position, in which the through flow of hydraulic fluid is blocked. However, even with these relatively small differences in pressure and the associated fluctuations, an opening is urgently required for operational reasons.
Hydraulic vehicle drives of this type belong to the prior art. As mentioned above, flushing spool valves which connect the low-pressure side, i.e. that side which transports the oil back to the variable displacement pump to the flushing-pressure limiting valve, are customary here. This flushing-pressure limiting valve is set to a lower pressure of the hydraulic fluid than the filling-pressure limiting valve of the filling pump, which is customarily present. This continuously produces an artificial leakage. The flushing spool valve is equipped, according to the prior art, with compression springs which, when the pressure between the high pressure side and the low-pressure side is the same, push the spool-valve piston into its neutral central position, in which no scouring takes place via the flushing-pressure limiting valve. By contrast, when there are clear differences in pressure between the high-pressure side and low-pressure side, such as those corresponding to an opening pressure of between 7 and 16 bar, the flushing spool valve opens and conducts the low-pressure side to the flushing-pressure limiting valve. In the normal driving mode these systems operate satisfactorily.
Difficulties arise, however, if the vehicle is no longer being driven by the internal combustion engine, but rather is being propelled in the overrun mode, also known as the negative mode. In this case, the vehicle mass propels the vehicle, e.g. in downhill travel, as a result of which the high-pressure side and low-pressure side of the closed hydraulic circuit change and the hydraulic motor operates as a pump and delivers a torque to the variable displacement pump. In this overrun mode, phases occur in which the high-pressure side and low-pressure side lie closely adjacent to each other, so that the spool-valve piston of the flushing spool valve remains in its neutral central position. In the case of very sturdy flushing spool valves which are configured to an opening pressure of 14 to 16 bar, operating pressures of +/−28 to 32 bar may occur without the flushing spool valve switching. Therefore, an exchange of oil from the closed circuit does not take place either. The oil heats up very strongly and may result in damage to the hydraulic motor. Tests using an additional temperature-regulating means or with the replacement of the compression springs by weaker compression springs have not led to any satisfactory result. On the contrary, severe pressure fluctuations have been found during operation, causing undesirable jarring through the entire vehicle.
It has namely turned out that, in the overrun mode, a relatively small volume of flushing flow is completely sufficient, provided that a rapid and easy response of the flushing spool valve is ensured even if the differences in pressure between the high-pressure and low-pressure line are small.
It is therefore a primary object of the present invention to provide a hydraulic spool valve that not only functions reliably at the customary high and constant differences in pressure, but also functions when there is a small and changing difference in the hydraulic pressure in the inlet bores.
It is therefore a further object of the invention to provide a hydraulic vehicle drive in such a manner that, even in the overrun mode, a satisfactory cooling of the hydraulic fluid is ensured.
These and other objects will be apparent to those skilled in the art.